1. Field of the Invention
The present invention relates to a positioning device for a table on which a semiconductor wafer is moved into an inspection position.
2. Background Information
An X.multidot.Y wafer stage or X.multidot.Y stage has been known, and such a stage moves an inspection area, which supports a wafer, for measuring extremely small displacements in a pattern formed on a semiconductor wafer.
Conventionally, X.multidot.Y wafer stages use an orthogonal structure of X and Y axes crossed by means of a ball screw, etc. This arrangement has a two-stage structure and its size is therefore large.
FIG. 1(A) is a schematic view for explaining the problems in such a conventional X.multidot.Y stage.
In this example, assume that a guide GF is moved perpendicular, that is, in the y direction to the planar of the figure, while being guided at its both ends. A stage T of W.sub.1 in weight is supported and guided by the guide GF, while being movable to right and left (in the x direction). The weight of the stage T is directly applied onto the guide GF, and therefore a certain amount of sagging occurs and for this reason, smooth and precise movement can not be expected.
Depending on the position of the stage T with respect to the guide GF, the load at the ends of the guide GF changes, and also the amount of sagging changes. There is also the problem that depending on the position of the stage T, the height of the stage T changes.
In order to observe the condition of the wafer surface supported by the stage T using a microscope, etc., if the height of the stage T differs, the focus must be adjusted.
As a mechanism for converting rotational motion into rectilinear motion in order to move an X.multidot.Y wafer stage or X.multidot.Y plotter, etc., a so-called ball screw type mechanism is known, in which the X.multidot.Y wafer stage, etc., which is rectilinearly guided and connected to a screw, is driven by rotating the screw. A moving mechanism using a wire is also known.
A conventional wire type moving mechanism is explained here together with the problem associated with it.
FIGS. 2(A) and 2(B) are schematic views for explaining the principle of operation of the conventional wire type moving mechanism. A wire 101 is passed around a pulley 111 and an idle pulley 104. Between the pulleys, the wire 101 is connected with a stage 109, and when the pulley 111 is rotated by a motor 108, the wire 101 is moved to thereby move the stage 109 along with the wire 101.
In order to avoid any slip between the pulley 111 and the wire 101, a certain tension is applied to the wire 101 and also the wire is wound around the pulley 111 several times.
As has been well known, by the rotation of the pulley 111, the wire 101 shifts a distance equal to a diameter of the wire 101 towards the side being taken up. The length of the pulley 111 is designed to include this shift.
This results in the displacement by the stage of an angle .theta. between the both ends. While being moved, this axial movement of the wire 101 causes a displacement of an object or expansion of a wire 101, and as the result precise moving is not expected.
For this reason, the conventional devices have been used only when some differences in the rotational number and the moving are allowed.
An inspection of a pattern formed on a wafer surface is generally made by means of an optical microscope. Adjustment of the height of a stage of the auto-focusing mechanism in the optical microscope has been made by means of a fine resolution lifting mechanism.
In order to control the height of the stage of the optical microscope using a pulse motor, etc. for adjusting the focusing, a fine resolution lifting mechanism of a stage is required.
Fine resolution lifting using a beam requires a beam having a large beam ratio.
But, for an inspection machine for semiconductor wafers, it is desirable that the entire size of the machine is made as small as possible.